U.S. patent number 8,638,831 [Application Number 13/760,222] was granted by the patent office on 2014-01-28 for optical pumping of a solid-state gain-medium using a diode-laser bar stack with individually addressable bars.
This patent grant is currently assigned to Coherent, Inc.. The grantee listed for this patent is Coherent, Inc.. Invention is credited to R. Russel Austin, Mark M. Gitin, David Schleuning.
United States Patent |
8,638,831 |
Schleuning , et al. |
January 28, 2014 |
Optical pumping of a solid-state gain-medium using a diode-laser
bar stack with individually addressable bars
Abstract
A diode-laser bar stack includes a plurality of diode-laser bars
having different temperature dependent peak-emission wavelengths.
The stack is arranged such that the bars can be separately powered.
This allows one or more of the bars to be "on" while others are
"off". A switching arrangement is described for selectively turning
bars on or off, responsive to a signal representative of the
temperature of the diode-laser bar stack, for providing a desired
total emission spectrum.
Inventors: |
Schleuning; David (Oakland,
CA), Gitin; Mark M. (Mountain View, CA), Austin; R.
Russel (La Veta, CO) |
Applicant: |
Name |
City |
State |
Country |
Type |
Coherent, Inc. |
Santa Clara |
CA |
US |
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Assignee: |
Coherent, Inc. (Santa Clara,
CA)
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Family
ID: |
45556143 |
Appl.
No.: |
13/760,222 |
Filed: |
February 6, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130148679 A1 |
Jun 13, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12852843 |
Aug 9, 2010 |
8391328 |
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Current U.S.
Class: |
372/50.12;
372/50.122; 372/34; 372/50.121 |
Current CPC
Class: |
H01S
5/06825 (20130101); H01S 5/041 (20130101); H01S
5/4018 (20130101); H01S 5/06804 (20130101); H01S
3/0941 (20130101); H01S 5/0428 (20130101); H01S
5/4043 (20130101) |
Current International
Class: |
H01S
3/04 (20060101); H01S 5/00 (20060101) |
Field of
Search: |
;372/34,36,50.12,50.121,50.122 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Non Final Office Action received for U.S. Appl. No. 12/852,843,
mailed on Aug. 15, 2012, 10 pages. cited by applicant .
Notice of Allowance received for U.S. Appl. No. 12/852,843, mailed
on Dec. 3, 2012, 9 pages. cited by applicant.
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Primary Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Morrison & Foerster LLP
Parent Case Text
PRIORITY
This application is a divisional of U.S. patent application Ser.
No. 12/852,843, filed Aug. 9, 2010.
Claims
What is claimed is:
1. A method of optically pumping a gain medium with a stack of
diode laser bars, wherein at least some of the diode-laser bars in
the stack have a different peak-emission wavelength at different
temperatures, with the peak-emission wavelengths being directly
dependent on the diode laser bar temperature, said method
comprising the step of: supplying current to at least one but less
than all the bars in the stack to generate laser output for
optically pumping the gain medium; monitoring the temperature of
the stack; and in response to the monitored temperature, changing
which bars in the stack receive current in order to better match
the wavelength emission of the stack to the peak absorption
wavelength of the gain medium.
2. The method of claim 1, wherein the gain-medium is Nd:YAG, having
a primary peak absorption at a wavelength of about 808 nanometers
with a lower secondary peak absorption at a wavelength of about 805
nanometers.
3. The method of claim 1, wherein the diode-laser bar stack is
sandwiched between first and second cooling members each having a
surface in thermal and electrical contact with the diode-laser bar
stack.
4. The method of claim 3 wherein the temperature of the stack is
monitored by temperature sensor.
5. The method of claim 4, wherein the temperature sensor is
embedded in one of the cooling members proximate the surface
thereof in thermal and electrical contact with the diode-laser bar
stack.
6. The method of claim 1 wherein said step of changing which bars
in the stack receive current is performed by a switching
arrangement cooperative with a current supply and arranged for
selectively turning on or off any one or more of the diode laser
bars from the current supply independent of the others.
7. A method of optically pumping a gain medium with a stack of
diode laser bars, with at least one of the diode laser bars in the
stack having a peak-emission wavelength that is significantly
different from the peak-emission wavelength of at least one of the
other bars, said method comprising; supplying current to at least
one but less than all the bars in the stack to generate laser
output for optically pumping the gain medium; and selectively
turning on or off any one or more of the diode laser bars from the
current supply independent of the others in order to better match
the wavelength emission of the stack to the peak absorption
wavelength of the gain medium.
8. The method of claim 7, wherein the gain-medium is Nd:YAG, having
a primary peak absorption at a wavelength of about 808 nanometers
with a lower secondary peak absorption at a wavelength of about 805
nanometers.
9. The method of claim 7 further including the step of monitoring
the temperature of the stack and wherein the step of selectively
turning on or off the diode laser bars is responsive to the
monitored temperature.
10. The method of claim 9, wherein the diode-laser bar stack is
sandwiched between first and second cooling members each having a
surface in thermal and electrical contact with the diode-laser bar
stack.
11. The method of claim 10 wherein the temperature of the stack is
monitored by temperature sensor.
12. The method of claim 11, wherein the temperature sensor is
embedded in one of the cooling members proximate the surface
thereof in thermal and electrical contact with the diode-laser bar
stack.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to solid-state lasers
pumped with diode-laser radiation. The invention relates to such
lasers wherein the diode-laser radiation is provided by a stack of
diode-laser bars.
DISCUSSION OF BACKGROUND ART
Radiation from a diode-laser or an array thereof is now extensively
used for optical pumping of lasers having a solid-state
gain-medium, in particular lasers wherein the wherein the
gain-medium is neodymium-doped yttrium aluminum garnet (Nd:YAG) or
neodymium-doped yttrium ortho-vanadate (Nd:YVO.sub.4). Diode-laser
radiation can be generated with efficiencies up to about 50% or
greater. The wavelength of the radiation (emission-wavelength) can
be selected, depending on the composition of the diode-laser
materials, to match the peak-absorption wavelengths of the
gain-media. The bandwidth of the radiation is comparable with the
FWHM bandwidth of absorption peaks in the absorption spectrum of
the gain-media.
A single diode-laser does not provide adequate power for pumping a
high-power-solid state laser. A two-dimensional array of
diode-lasers is required to pump really high-power lasers, for
example, lasers having a peak power of about 200 W or greater. Such
lasers are typically operated in a repetitively pulsed manner by
driving the diode-laser array in a repetitively pulsed manner.
A two-dimensional array of diode-lasers is typically made by
stacking one-dimensional diode-laser arrays known in the art as
diode-laser bars. A diode-laser bar includes a plurality of
individual diode-lasers (emitters) formed in semiconductor layers
epitaxially-grown on a single elongated substrate. The substrate or
bar (cut from a disk substrate) typically has a length of about 1
centimeter (cm), a width of about 1 millimeter (mm), and a height
of about 140 micrometers (.mu.m) or less. The emitters emit along a
propagation-axis in the width direction of the bar, and have
slow-axis (low beam-divergence) in the length direction of the bar
and a fast-axis (high beam-divergence) perpendicular to the slow
axis, i.e., in the height direction of the bar. Current is
typically passed through the bars electrically connected in series,
with emitters in any one bar connected in parallel
One drawback of diode-lasers is that the emission wavelength of a
diode-laser or diode-laser bar is relatively strongly dependent on
the diode-laser temperature. By way of example, for GaAs P/InGaAs
diode-lasers, peak emission wavelength varies by about 0.3
nanometers (nm) per .degree. C. The diode-laser bar temperature,
absent effective cooling, depends, inter alia, on the current
passed through the diode-laser bar and the pulse-duration with
which the diode-laser bar is driven. In a diode-laser bar stack
(two-dimensional diode-laser array) the total emission from the
stack is brighter the closer together the one dimensional-emitter
arrays of the bars are in the stack.
With a close-stacking, providing equal cooling of the bars is
extremely difficult if not impossible, as only outermost bars of
the stack can be contacted by massive cooling members. This means
that bars in the center of a stack will get hotter than bars at or
near the top or bottom of the stack making wavelength control of
individual bars very difficult.
In U.S. Patent Application Publication No. 2010/0183039, assigned
to the assignee of the present invention, and the complete
disclosure of which is hereby incorporated herein by reference, a
diode-laser bar stack is described in which diode-laser bars are
selected with different emission wavelengths at the same
temperature and located in the stack such that at a nominal
operating condition of the stack, where the bars reach different
temperatures, the total emission of the stack has a bandwidth
significantly greater than that of any one bar in the stack. In
this way, it can be arranged that the absorption peak of a
gain-medium being pumped can lie within the total emission
bandwidth at any anticipated range of operating conditions
(temperatures) of the stack. This eliminates a need to control the
stack temperature by active means.
FIG. 1 schematically illustrates a prior-art diode-laser pumping
arrangement 10. The arrangement is described in detail in the
above-referenced patent publication. The arrangement includes a
stack 11 of six diode-laser bars 12A, 12B, 12C, 12D, 12E, and 12F.
Each bar includes a heterostructure 14 grown on a substrate 16.
Diode-laser emitters (not shown) are designated within the
heterostructure, as is known in the art. The fast-axis, slow-axis,
and emission-direction (propagation-axis) are indicated in FIG. 1
by axes Y, X, and Z, respectively. The bars are soldered one to the
next, with the epitaxial-layer side of one bar soldered to the
substrate side of an adjacent bar such that the emitters are
connected in series-parallel.
Stack 11 is sandwiched between a heat-sink member 18 and a
heat-sink member 20, with both heat-sink members being supported on
a base 22. There is a space 24 between the stack and the base. The
epitaxial side 14F of bar 12F is in thermal contact with heat-sink
member 20. The substrate side 16A of bar 12A is in thermal contact
with heat-sink member 18. The diode-laser bars are in thermal
contact with each other, with the epitaxial side of one in thermal
contact with the substrate side of the next except of bar 12F.
Heat-sink members 18 and 20 are insulated from base 22 by
insulating layers 17 and 19 respectively. Current from a pulsed
power supply (not shown) for driving the stack is connected to the
stack by attaching a positive lead to heat-sink member 20 and a
negative lead to heat-sink member 18.
FIG. 2 is a graph schematically illustrating the calculated total
emission spectrum (solid curve) of the stack of FIG. 1 overlaid
with the absorption spectrum (dashed curve) of Nd:YAG. It is
assumed that the stack is driven in a pulsed mode with a pulse
duration of 250 microseconds (.mu.s), a pulse-repetition frequency
(PRF) of 2 Hertz (Hz) and an average power of 200 Watts (w) per
bar. It is assumed that the nominal emission-wavelengths of bars
12A, 12B, 12C, 12D, 12E, and 12F are 801.71 nm, 808.10 nm, 804.49
nm, 808.10 nm, 804.43 nm, and 801.05 nm, respectively.
The nominal bandwidth of the total emission from the stack is about
10.0 nm, with the center wavelength close to the 808 nm absorption
peak of the Nd:YAG absorption spectrum. In theory at least, the
average temperature of the six bars could vary by about
.+-.10.degree. C. from the temperature providing the emission
spectrum of FIG. 2, with the 808 nm absorption peak still remaining
within the total emission bandwidth.
A particular drawback of this prior-art stack arrangement is that
only a fraction of the total emission power provided by the stack
(about 25% in the overlay of FIG. 1) is absorbed by the
gain-medium. This considerably reduces the electrical to optical
pumping efficiency of the stack. There is a need to provide a
diode-laser bar stack for optical pumping that is capable of
operating over a relatively wide uncontrolled temperature range but
will deliver radiation in only a relatively narrow wavelength range
around a gain-medium absorption peak of interest.
SUMMARY OF THE INVENTION
In one aspect of the invention, electro-optical apparatus comprises
a plurality of diode-laser bars positioned one on another to form a
diode-laser bar stack, with at least one of the diode-laser bars
having a peak-emission emission wavelength that is significantly
different from the peak-emission wavelength of at least one of the
others. The diode-laser bar stack is arranged such that each of the
diode-laser bars can be separately powered independent of the
others.
The term significant, here, means that the wavelength difference is
greater than would be encountered in normal manufacturing
tolerances for diode-laser bars. When provided with a power supply
and an appropriate switching arrangement, the stack can be driven
with a selected one or more of the bars powered and the remainder
not powered.
In an example of the apparatus used in conjunction with the
switching arrangement for pumping a laser gain-medium, each of the
plurality of diode-laser bars has a different peak-emission
wavelength at a given temperature. The peak-emission wavelengths
are directly dependent on the diode-laser bar temperature, and the
peak-emission wavelengths of the diode-laser bars are selected such
that as the temperature of the diode-laser bar stack increases
through a predetermined range at least one of the diode laser bars
will have a peak-emission wavelength falling within a predetermined
absorption band in the absorption spectrum the gain-medium. The
temperature of the laser bar stack is monitored and the monitored
temperature is used by the switching arrangement to determine which
of the diode-laser bars should be turned on and which should be
turned off at any given temperature in the range.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, schematically illustrate a preferred
embodiment of the present invention, and together with the general
description given above and the detailed description of the
preferred embodiment given below, serve to explain principles of
the present invention.
FIG. 1 schematically illustrates a prior-art stack of six
diode-laser bars connected in series with a current-supply and
wherein at least one of the diode-laser bars has an emission
wavelength different from one or more of the others.
FIG. 2 is a graph schematically illustrating the absorption
spectrum of Nd:YAG overlaid by the emission spectrum of an example
of the diode-laser bar stack of FIG. 1 wherein the stack is driven
in a pulsed mode at 200 Watts per bar output, with the bars having
nominal emission wavelengths of 801.71 nm, 808.10 nm, 804.49 nm,
808.10 nm, 804.43 nm, and 801.05 nm.
FIG. 3 is three-dimensional view schematically illustrates one
preferred embodiment of a diode-laser bar stack in accordance with
the present invention including a fast-axis stack of five
diode-laser bars with electrical contact strips therebetween,
electrical leads for individually addressing each of the
diode-laser bars and a temperature sensor for monitoring the
temperature of the diode-laser bar stack.
FIG. 4 is an electrical circuit diagram schematically illustrating
an electrical representation of the diode-laser bar stack of FIG. 4
with the five diode-laser bars connected in series and switching
electronics responsive to a signal from the temperature sensor of
FIG. 3, the switch electronics including 5 MOSFETs connected in
series with each other with each of the MOSFETs connected in
parallel with a corresponding one of the diode-laser bars.
FIG. 4A is an electric diagram similar to the diagram of FIG. 4 but
illustrating a current path through the MOSFETs and diode-laser
bars when two of the five diode-laser bars are turned on and the
remainder are turned off.
FIG. 5 is a graph schematically illustrating a measured
total-emission spectrum of one example of the diode-laser bar stack
of FIG. 3 and measured spectra of each one of the diode-laser bars
in the stack.
FIG. 6 is a graph schematically illustrating the measured
peak-emission wavelength as a function of temperature of each of
the diode-lasers bars in the example of FIG. 5.
FIG. 7 is a graph schematically illustrating the individual
diode-laser bar spectra of FIG. 5 overlaid with the absorption
spectrum of Nd:YAG.
FIG. 8 schematically represents one example of a look-up table in
accordance with the present invention used to operate the MOSFETs
of FIG. 4, responsive to a temperature signal from the sensor of
FIGS. 3 and 4, for turning on and off selected ones of the
diode-laser bars in the exemplary stack of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
Continuing with reference to the drawings, wherein like components
are designated by like reference numerals, FIG. 3 schematically
illustrates one preferred embodiment 30 of an individually
addressable diode-laser bar stack in accordance with the present
invention. This diode-laser bar stack includes five diode-laser
bars 12A, 12B, 12C, 12D, and 12E, having epitaxially grown
semiconductor multilayers 14A, 14B, 14C, 14D, and 14E. The
diode-laser bars are stacked in the fast-axis (here, the Y-axis)
direction. Stack 30 is similar to the prior-art stack 10 of FIG. 1
but with important differences as follows.
Between adjacent diode-laser bars in the stack 30 is an electrical
contact strip 32. This can be formed from one or more layers of
electrically and thermally conductive material including solder
layers. There are four electrical contact strips 32. The
diode-laser bars and contact strips are sandwiched between two
passive cooling-members 18 and 20. Upper and lower ones of the
diode-laser bar are in thermal and electrical contact with a
surface of cooling members 20 and 18, respectively.
Each contact strip 32 has an electrical lead attached thereto. The
leads attached to the contact strips are designated in FIG. 3 as
leads 34B, 34C, 34D, and 34E. These leads make contact to the
electrical junction between diode-laser bars 12A and 12B, 12B and
12C, 12C and 12D, and 12D and 12E, respectively. There is an
electrical lead 34A attached to passive cooling-member 20, and an
electrical lead 34F attached to passive cooling-member 18.
Together, these leads allow each diode-laser bar to be individually
addressed and powered electrically. The leads can be connected with
control circuitry for the diode-laser bar stack, a description of
one example of which is provided further hereinbelow with reference
to FIG. 4.
Means are provided for monitoring the temperature of the
diode-laser bar stack. In the embodiment of FIG. 3, the temperature
monitoring means are provided by a temperature sensor 38, embedded
in cooling member 18 near that surface of the cooling member that
is in thermal contact with diode-laser bar 12E. One preferred
temperature sensor is a high-accuracy low-power digital temperature
sensor model number TMP 112 available from Texas Instruments Inc.
of Dallas, Tex. DC power is supplied to the sensor from a DC Power
supply (not explicitly shown in FIG. 3) via a lead 52. A serial bus
36 transmits a digital signal from the temperature sensor to the
above-referenced control circuitry.
FIG. 4 is an electrical circuit diagram schematically illustrating
an electrical representation of the diode-laser bar stack of FIG. 4
with the five diode-laser bars connected in series (stack 30) and
switching electronics (control circuitry) 40 responsive to a signal
from the temperature sensor of FIG. 3. Each diode-laser bar is
represented by a single electrical-diode symbol for convenience of
illustration. In practice each diode-laser bar includes a plurality
of spaced-apart diode-laser emitters with the emitters connected in
parallel.
Switching electronics 40 includes MOSFETs 44A, 44B, 44C, 44D, and
44E connected in series with each other across a current-limited
power supply 28 which provides current for driving the diode-laser
bars. Each of the MOSFETs is connected in parallel with a
corresponding one of the diode-laser bars, which are also serially
connected across power supply 28. MOSFETs 44A, 44B, 44C, 44D, and
44E are driven by MOSFET drivers 48A, 48B, 48C, 48D, and 48E,
respectively, the MOSFET drivers are powered by a MOSFET driver
power supply 56. One suitable MOSFET driver is a model number LTC
1693 available from LINEAR Technology Incorporated of Milpitas,
Calif.
Circuitry 40 includes a Microcontroller with erasable programmable
memory (EEPROM) designated, here by a single functional block 42.
One suitable microcontroller is a PIC24F16KA102 microcontroller
available from Microchip Technology Inc. of Chandler, Ariz. The
EEPROM stores a look-up table which, based on the temperature
monitored by sensor 38 to provide a digital signal for selectively
turning the MOSFETS on or off as required. An example of the
look-up table is described further hereinbelow. MOSFET drivers 48A,
48B, 48C, 48D, and 48E are connected to the microcontroller by
leads 46A, 46B, 46C, 46D, and 46E, respectively. A DC power supply
50 supplies power to the microcontroller via lead 54 and to the
temperature sensor via lead 52.
If a diode-laser bar is to be turned off (disabled) the MOSFET
connected in parallel thereto must be turned on. If a MOSFET is to
be turned on the microcontroller sends "high" signal (digital 1) to
the appropriate MOSFET driver. On receipt of this signal the MOSFET
driver applies a voltage V to the gate of the MOSFET. The Voltage
must be sufficiently high that the MOSFET is turned completely on
and diverts sufficient current away from the diode-laser bar that
current passing through the diode-laser bar is below threshold and
the diode-laser bar will not emit radiation. If a diode-laser bar
is to be turned on (enabled) the parallel connected MOSFET must be
turned off. In order to turn a MOSFET off the microcontroller sends
a "low" signal (digital 0) to the MOSFET driver and the driver does
not apply any voltage to the MOSFET gate. In this case the MOSFET
does not divert any current from the diode-laser bar and sufficient
current can pass the through the diode-laser bar such that the bar
emits laser radiation.
An example of the above-described switching is schematically
illustrated in FIG. 4A, which is similar to the circuit diagram of
FIG. 4, but wherein the current path through the MOSFET/diode-laser
bar network is depicted (by three-point bold lines) for a case
where diode-laser bars 12B and 12D are turned on and the remainder
are turned off. The "on" (enabled) diode-laser bar symbols are
shaded and the "off" diode-laser bars are unshaded. Emitted
radiation is indicated by bold arrows E. Driver signals are
indicated as 1 or 0.
It should be noted above that other circuit components may be
required to optimize function if parasitic inductance or the like
are encountered. Those skilled in the electronic art will recognize
which components would be needed to deal with such parasitic. Those
skilled in the art will also recognize, without further
illustration or detailed description, that the microcontroller
could be set up to monitor for undesirable conditions and act to
put the system into a safe configuration and warn that the system
is not functioning correctly.
Note that the current is delivered from the current supply in a
pulsed manner, with some predetermined pulse duration and duty
cycle. The nominal temperature of the diode-laser bar stack
(monitored by temperature sensor 38) will depend on the magnitude
of the current, the pulse duration, the duty cycle, and which
combination of diode-laser bars is "on", i.e., emitting laser
radiation. There may be some temperature difference between
diode-laser bars that are "on" depending on the position of those
bars in the stack, with a bar further from a cooling member than
another having the higher temperature of the two.
FIG. 5 is a graph schematically illustrating a measured
total-emission spectrum (bold curve) of one example of the
diode-laser bar stack of FIG. 3 wherein each diode-laser bar has a
different peak-emission wavelength from that of the others. The
graph also depicts the measured spectrum (fine curve) of each one
of the diode-laser bars in the stack. In this example diode-laser
bars 12A, 12B, 12C, 12D, and 12E have peak-emission wavelengths
(peak .lamda.) of 810.66, 807.88, 804.52, 810.46, and 798.42,
respectively at a temperature of about 30.degree. C., i.e., the
peak emission wavelengths are between about 795 nm and about 815
nm. The corresponding full-width at half-maximum (FWHM)
emission-bandwidths is 2.04, 2.18, 2.48, 2.17, and 2.29,
respectively, i.e., the emission bandwidths are between about 2.0
nm and about 2.5 nm. The peak-emission wavelengths are spaced apart
by between about 2.5 nanometers and about 3.1 nanometers.
FIG. 6 is a graph schematically illustrating measured curves
(system calibration curves) of peak-emission wavelength as a
function of temperature. An elongated, rectangular, shaded area on
the graph represents a peak emission of 808 nm with a FWHM of 2.5
nm.
FIG. 7 is a graph schematically illustrating the individual
diode-laser bar spectra of FIG. 5 (solid fine curves) overlaid by
the Nd:YAG absorption spectrum (bold dashed curve) of FIG. 2. It
can be seen from FIG. 7 that the 808 nm primary absorption peak of
the absorption band and the 805 nm shoulder portion (secondary
peak) of the absorption band only just fit within the emission
spectrum of diode-laser bar 12B at the measurement temperature.
FIG. 8 is a representation of a look-up table used in circuit block
42 of the electrical circuitry of FIG. 4 to generate the
above-discussed five-bit digital word that determines which of the
MOSFETS are turned on (and corresponding diode-lasers turned off)
by voltages are applied to the gates of the MOSFETS. The table is
produced using the peak-wavelength as a function of temperature
calibration curves of FIG. 5. It is determined that two adjacent
diode-lasers should be on wherever possible. This is in order to
accommodate the close fit of the emission curve of any diode-laser
bar emission spectrum with the shouldered 808 nm absorption peak
and any temperature measurement inaccuracy due to above-discussed
position-related temperature differences between adjacent bars.
Having two adjacent diode-laser bars "on" also provides that there
is some absorption in the 808-nm peak when switching from one
adjacent pair to the next as temperature rises or falls, thereby
accommodating a stepwise switching at the predetermined temperature
intervals.
In the look up table of FIG. 8, digital zero (0) causes zero
voltage to be applied to a MOSFET gate, such that all current flows
through the corresponding diode-laser bar, thereby causing the bar
to emit laser-radiation. This diode-laser bar "on" condition is
indicated by the shaded blocks of the table including the digital
zero. A digital one (1) causes a corresponding MOSFET driver to
apply voltage V to the corresponding MOSFET gate so that the MOSFET
draws sufficient current to effectively short-out the corresponding
diode-laser bar, such that the diode-laser bar can not emit
radiation.
In the look-up table of FIG. 8, while the signal from the
temperature sensor indicates a stack temperature of less than or
equal to 25.degree. C. the table will provide digital word 00111
which would enable diode-laser bars 12A and 12B and disable
diode-laser bars 12C, 12D, and 12E. If the sensor-signal indicates
temperatures greater than 25.degree. C. up to 35.degree. C., the
digital word will be 100 .mu.l, whereby diode-laser bars 12B and
12C will be enabled and diode-laser bars 12A, 12D and 12E will be
disabled. If the sensor-signal indicates temperatures greater than
35.degree. C. up to 45.degree. C., the digital word will be 11001,
whereby diode-laser bars 12C and 12D will be enabled and
diode-laser bars 12A, 12B and 12E would be disabled. If the
sensor-signal indicates temperatures greater than 45.degree. C. up
to 55.degree. C., the digital word will be 11100, whereby
diode-laser bars 12D and 12E will be enabled and diode-laser bars
12A, 12B and 12C will be disabled. If the sensor-signal indicates
temperatures greater than 55.degree. C., the digital word will be
11110, whereby only diode-laser bar 12E will be enabled and
diode-laser bars 12A, 12B, 12C, and 12D will be disabled. The
terminology "enabled" and "disabled", as used here, recognizes that
a diode-laser bar can only deliver laser radiation (if enabled)
when current supply 28 is delivering a current pulse.
It is emphasized here that although the diode-laser bars are
supplied with current from a common power supply they can be
individually powered independent of the others. This is because any
one of the diode-laser bars is only supplied with current when the
corresponding MOSFET does not bypass the current around the
diode-laser bar, effectively disconnecting the bar from the supply.
This is illustrated amply in the example of FIG. 4.
Those skilled in the art will recognize from the description of the
present invention provided above that more or less than five
diode-laser bars could be included in the inventive stack with
different wavelength spacing; with more than one bar at any given
wavelength; and with different combinations of diode-laser bars
enabled for any given temperature range. The diode-laser bars do
not need to be arranged in the stack in increasing or decreasing
wavelength order as described. Any such arrangement may be
implemented without departing from the spirit and scope of the
present invention.
Those skilled in the art will also recognize that while the present
invention is described in terms of a digital operation of a
multi-wavelength diode-laser bar stack, with some combination of
bars fully enabled and others fully disabled, that operating mode
should not be considered limiting. It would be possible and may
even advantageous to operate the inventive stack in an analog or
digital and analog mode with different "on" bars emitting radiation
at selectively different power. This could be done, for example, to
provide a total emission spectrum to shape to match the spectral
shape of a gain-medium absorption peak.
The MOSFET switching system of FIG. 4 lends itself to such analog
operation as the drain current of such MOSFETs is dependent on the
gate voltage. As the gate voltage is increased the saturated
(drain-voltage independent) drain current increases. Increasing the
drain current of a MOSFET decreases the current through the
corresponding parallel-connected diode-laser bar thereby decreasing
the power of radiation emitted by the diode-laser bar. Providing
power variation in a bar would require making voltages V applied to
the MOSFET gates selectively variable. Because of this, such an
analog operation would however require a more extensive calibration
and a more complicated MOSFET driving arrangement than that
described above for digital operation. Based on the description of
the digital driving arrangement provided herein, the design of such
an analog driving arrangement would be within the capability of one
skilled in the electronic arts.
It should also be noted that while a monitored stack-temperature is
used in the above-described embodiment to determine which bars
should be enabled and which disabled, it is possible, in theory at
least, to use some other monitored value to determine this, while
still using a similar switch arrangement for actual operation. Such
a value could be laser output power or absorbed pump-power (which
would require monitoring pump power onto and out of the
gain-medium).
Absent a failure of one or more diode-laser bars or a solid-state
laser component, however, changes in laser output power or absorbed
pump-power values would normally result from a change in
temperature, and accordingly the emission spectrum, of the
diode-laser bar stack. If, for example, laser output power were
monitored in addition to temperature, the output power could be
compared before and after a temperature-dictated switching of
diode-laser bars. If the output power did not rise as anticipated
corresponding to the diode-laser switching, it would be reasonable
to suspect a malfunction of one or more of the diode-laser bars.
This would provide input for above discussed additional functions
of the microcontroller.
In summary, the present invention is described above in terms of a
preferred embodiment. The invention, however, is not limited to the
embodiment described and depicted. Rather the invention is limited
only to the claims appended hereto.
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